Field of the Invention
The present invention relates to methods and apparatuses for controlling a machine tool in which a main spindle on which a tool is mounted and a table that holds a workpiece are moved relative to each other by two or more translation axes that are not perpendicular to each other and at least one rotation axis to machine the workpiece with the tool, and an error in the position of the tool with respect to the workpiece due to a geometric error is corrected to calculate a command value for controlling the translation axis.
Description of Related Art
An example of such a machine tool is shown in FIG. 6. FIG. 6 is a schematic view of a 5-axis control machining center 101 having three translation axes and two rotation axes. A spindle head 102 has two translational degrees of freedom relative to a bed 103 by X- and Z-axes serving as translation axes perpendicular to each other. A table 104 has a single rotational degree of freedom relative to a cradle 105 by a C-axis serving as a rotation axis. The cradle 105 has a single rotational degree of freedom relative to a trunnion 106 by an A-axis serving as a rotation axis. The A-axis and the C-axis are perpendicular to each other. The trunnion 106 has a single translational degree of freedom relative to the bed 103 by a Y-axis serving as a translation axis perpendicular to the X- and Z-axes. Each axis is driven by a servomotor (not shown) that is controlled by a numerical control apparatus (not shown). A workpiece is fixed to the table 104, a tool (not shown) is mounted on the spindle head 102 and rotated. The workpiece is thus machined by controlling the relative positions of the workpiece and the tool.
Factors that affect motion accuracy of the 5-axis control machining center 101, for example, include geometric errors between the axes such as an error in the center position of the rotation axis (shift from its intended position) and an error in inclination of the rotation axis (squareness or parallelism between the axes). As such a geometric error decreases motion accuracy of the 5-axis control machining center 101 and decreases machining accuracy of workpieces, it is required to reduce the geometric error by adjustment. However, it is difficult to eliminate the geometric error by adjustment, and control of correcting the geometric error is performed to obtain accurate machining.
Such a method as disclosed in Japanese Patent Application Publication No. 2004-272887 (JP 2004-272887 A) proposes a method wherein a geometric error is corrected. In the method disclosed in JP 2004-272887 A, the position of the distal end point of a tool is converted to the position of each translation axis in view of a geometric error of a machine tool, and these positions of the translation axes are used as command values for controlling the translation axes to correct a position error of the distal end point of the tool due to the geometric error.
Japanese Patent Application Publication No. 2009-104317 (JP 2009-104317 A) discloses a method for calculating command values for controlling translation axes. In this method, a deformation error associated with operation of a machine tool, a positioning error that is caused according to a command position for the translation axis, and an error that is caused by thermal displacement due to heat generation etc. of each element of the machine tool are regarded as a geometric error. Correction values for the translation axes calculated based on the geometric error are added to command values for the translation axes. The command values for controlling the translation axes are thus calculated.
However, the methods disclosed in JP 2004-272887 A and JP 2009-104317 A are intended for those machine tools in which the X-, Y-, and Z-axes serving as translation axes are perpendicular to each other. Accordingly, with these methods, a geometric error of a machine tool including two or more translation axes that are not perpendicular to each other cannot be corrected, and a command value for the translation axis cannot be calculated.